Friday, October 1, 2010

OCC Brings Tropical to Charleston, SC

OCC's Doug Gaffney will be presenting a paper with Dr. Kelly Legault at the American Shore and Beach Preservation Association conference in Charleston, SC. The paper is entitled "Design and Function of Very Low Profile Groins" and will present findings from projects in Florida and the British Virgin Islands. The conference will take place from October 12-15 http://www.asbpa.org/conferences/conf_fall_10.htm

Similar to traditional groins, very low profile (VLP) groins are shore perpendicular structures that are intended to modify the beach by locally adjusting sediment transport. In contrast to traditional groins, VLP groins, designed to follow the natural slope of the beach, easily overtop and bypass sediment due to their low design crest elevation. As a result, the amount of sand retained within the groin field is modest and the impact to downdrift beaches is greatly reduced. Further, beach profile variability is diminished relative to non-engineered beaches because VLP groins tend to stabilize the subaerial beach.

This paper will investigate the length of VLP groins with respect to the cross-shore location of the surf zone, presence of offshore sand bars, water depth, and the crest elevation of the groin relative to the tide range and average wave height. Three case histories will be described, all of which utilized sand-filled geotextile tubes or bags as the primary construction material. The first installation was at Stump Pass State Park on the west coast of Florida in 2005. Three VLP groins were designed with short spacing, short length and small geotextile tube circumferences. These groins were tested by a brush with Hurricane Katrina, and the post storm response was excellent. The second installation in 2006 was a continuation of the groin field at Stump Pass. The crest elevation of the VLPs was significantly higher than the first set of tubes, resulting in a modified beach morphology that one would typically associate with traditional groins. The downdrift beach exhibited a lower elevation than the updrift beach, and the shoreline was recessed. Factors such as an equilibrating beachfill and high background erosion rate contributed to this observed condition. The third installation was placed in 2010 in the British Virgin Islands. The VLP groins were constructed of 15 foot long sand bags, rather than geotextile tubes, but otherwise retained many of the design features from the previous installations. The project performed excellently during Hurricane Earl.